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Add EitherOps#leftMapOrKeep and EitherOps#leftFlatMapOrKeep #4638

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Merged
merged 4 commits into from
Jul 23, 2024
Merged

Add EitherOps#leftMapOrKeep and EitherOps#leftFlatMapOrKeep #4638

merged 4 commits into from
Jul 23, 2024

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@danicheg
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@danicheg danicheg commented Jul 20, 2024

These are companions to Functor#mapOrKeep and cats.Monad#flatMapOrKeep, but for the left-hand side of Either[A, B].

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satorg previously approved these changes Jul 20, 2024
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Thank you!

satorg
satorg reviewed Jul 20, 2024
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core/src/main/scala/cats/syntax/either.scala Outdated
def leftMapOrKeep[AA >: A](pf: PartialFunction[A, AA]): Either[AA, B] =
eab match {
case Left(a) => Left(pf.applyOrElse(a, identity[AA]))
case r @ Right(_) => r
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@satorg satorg Jul 20, 2024 •
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I wonder if on a byte-code level this could be more performant:

case r : Right[B] => r

because there's no extractor to be called nor intermediate Option value to be created in opposite to the former case.

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@danicheg danicheg Jul 21, 2024

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Interestingly, I never thought that x @ Foo(_) might have any significant distinction from x: Foo.

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satorg commented Jul 20, 2024 •
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It totally makes sense, thank you!

I've realized - perhaps it makes sense to add leftMapOrKeep to cats.Bifunctor. That way we could get it for many other bi-parameter types, not just Either.

Things seem more complicated for leftFlatMapOrKeep though. Looks like there's no appropriate type class where leftFlatMapOrKeep could be added.

In fact, there's Bimonad in Cats, but it has a completely different meaning there, which I find quite confusing.

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danicheg commented Jul 21, 2024

I've realized - perhaps it makes sense to add leftMapOrKeep to cats.Bifunctor. That way we could get it for many other bi-parameter types, not just Either.

Agreed. Are you arguing we should do it as a drop-in replacement instead of adding EitherOps#leftMapOrKeep? Apart from the existence of the Bifunctor[Either] instance, there are also EitherOps#leftMap and its companion EitherOps#leftFlatMap.

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satorg commented Jul 21, 2024

I think it wouldn't hurt if there were leftMapOrKeep in both Either syntax and Bifunctor, similar to leftMap.
Moreover, calling to leftMap through Bifunctor on Either incurs quite a bit of overhead currently. It could be alleviated a little if we had a specialized version of leftMap in the Bifunctor[Either] implementation. But for now it is not there and it is a completely different story.

Also I feel this PR would be better off if it stays focused on the Either syntax only. Bifunctor could be addressed in a separate work.

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danicheg commented Jul 22, 2024

@satorg wouldn't you mind submitting yet another review?

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Thank you!

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danicheg commented Jul 23, 2024

@satorg thanks!

@danicheg danicheg merged commit aee3702 into typelevel:main Jul 23, 2024
@danicheg danicheg deleted the either-syntax branch July 23, 2024 05:20
@danicheg danicheg mentioned this pull request Jul 27, 2024
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